1. Field of the Invention
This invention relates to an overvoltage protector for use with a data cable and more particularly to such a device which is designed to be connected between the equipment to be protected and the data cable and utilize the equipment's chassis ground as the ground reference for surge suppression.
2. Description of the Prior Art
Mainframe computers, minicomputers and other similar data processing equipment have proliferated in the office environment. It has been quite common for such equipment to be connected to terminals usually located in the same office building for the purposes of data transmission. The data processing equipment typically includes an input/output port which meets the EIA's RS-232C specification ("the RS-232C port"). The connection may then be accomplished by using data communication cables ("data cables") to connect the RS-232C port of one unit either directly or through some intermediate piece of equipment to the RS-232C port of another unit.
Surges may appear on such data cables by any one of a number of causes. A surge may be induced on a data cable by lightening, AC power line disturbances, electrostatic discharge or even as a result of the explosion of an atomic device above the atmosphere. The surge appears on the data cable as a rising voltage. The rate at which the surge voltage rises depends on its cause. For example, the surge voltage caused by the atmospheric explosion of an atomic device, also known as electromagnetic pulse, rises at an extremely fast rate. No matter what the source is, it is desired to suppress all such surge voltages before they can cause damage to the integrated circuits of the equipment connected to the data cable.
Various types of surge suppression devices are well known in the art. They may either be solid state devices such as bipolar avalanche diodes, or gas discharge tubes. In general solid state devices respond relatively fast to surges, i.e. they limit overvoltages to a relatively low value, but cannot carry appreciable amounts of current. Gas discharge tubes generally respond more slowly to overvoltages than solid state devices, but can carry appreciable amounts of current. U.S. Pat. Nos. 4,104,693 and 4,491,893 both of which are assigned to the assignee as is the present invention show various examples of gas discharge tubes. The tube described in the '893 patent is capable of responding to electromagnetic pulses.
A surge suppression device whether of the solid state or gas discharge type suppresses a surge by conducting the surge to ground. In the past when such surge suppression devices were used for data cables the ground connection was obtained by using a ground lead external to the protector housing. The lead is meant to be connected to the ground of the AC power line. Such leads are not only inconvenient to connect but also have inductance associated therewith. The inductance interferes with the surge suppression as it opposes the sudden flow of surge current to ground.
It is desirable, therefore, to provide protection for surges induced on data cables. In particular, it is desirable that such a protector include a ground reference which utilizes the chassis ground of the equipment to be protected to thereby minimize any inductive effects. It is also desirable that such surge protection be relatively easy to provide and connect to the data cable and equipment to be protected. It is also desirable that such surge protection be provided in the form of a surge protector which connects to both the data cable and the RS-232C port of the equipment. It is also desirable to provide protection for such surges which have extremely fast rise times.